1. Field of the Invention
The present invention relates to a holding device for a glazing pane heated in a horizontal position to deformation temperature, the device having a downwardly facing contact surface, against which the glazing pane is held by differential pressure acting between the lower surface and the upper face of the glazing pane.
2. Description of the Related Art
Holding devices of this type are used in various processes for the production of curved glazing panes, especially the production of curved automobile glazing panes, in order to hold the glazing panes, heated in a once-through furnace to bending temperature, by suction action during the transfer from a horizontal conveyor track to a transporting ring.
A related method for the bending or curving of glazing panes, in which a holding device of this type is used, is described, for instance, in EP-B1 0 003 391 and 0 005 306. In this bending process, the glazing panes which have been heated to bending temperature are lifted off the conveying path by means of a usually plane suction plate and are brought by this suction plate above an annular mold. When the suction plate together with the glazing pane is situated above the annular mold, the suction in the suction plate is cancelled and the glazing pane falls onto the annular mold and seats itself down, under the action of the kinetic energy of descent and its self-weight, onto the annular mold.
In a further related bending process, the holding device is also constructed as a suction mold. In this case, the contact surface of the holding device has a curved shape, so that the glazing pane, as it is sucked onto the contact surface, either undergoes a prior bending or is already bent into its final shape (DE 20 00 271, FR 20 85 464). In this process, the glazing pane, bent on the holding device, is then laid on a support ring having the same peripheral shape, or it is pressed onto the support ring by the holding device before being laid on the support ring and is thereby bent into its final form.
A method of bending glazing panes by the use of the initially named holding device is furthermore known, in which the glazing pane, heated to bending temperature, is lifted by means of a vertically upwardly directed hot gas stream off the conveying path and is held against the holding device (DE-35 23 675). In this case the holding device is not constructed as a suction mold, but consists of a metal mold having a closed surface, against which the glazing pane is pressed by the excess pressure of the upwardly flowing gas stream.
In all the known methods, in which such a holding device is used, the glazing pane is transferred from the holding device onto a support ring, by which the glazing pane is transferred into a cooling station. As a rule, the bent glazing pane is thermally toughened or quenched in the cooling station by the blowing-on of cooling air jets for the production of single-pane safety glass.
In the period between the transfer onto the support ring and the cooling of the glazing pane below its deformation temperature, the glazing pane deforms under the effect of its self-weight, by sagging within the support ring. This deflection or sagging is especially large in the first named bending process, in which the glass pane is bent by the kinetic energy of descent.
To reduce the amount of the sag or deflection within the support ring, which is also termed transverse bending, it is known to provide the contact surface of the holding device with a corresponding concave counter-bend (EP 0 003 391). This counter-bending in the holding device should be so constructed that it compensates the deflection or sag within the support frame.
In practice, the amount of transverse bend under the effect of the self-weight of the glazing pane is not always the same, but changes for instance as a function of the temperature of the glazing pane, of the dwell time of the glazing pane on the support ring and, possibly also, of further process parameters. In the known holding devices, however, the counter-bending for the purpose of compensating the transverse bend cannot be modified, because it is incorporated into the rigid surface of the supporting device, which usually is of a ceramic material. The forming of the counter-bend is, furthermore, an expensive and complex process, so that for this reason alone the provision of a counter-bend in the contact surface of the holding device is frequently dispensed with.